Power supply devices of the aforesaid kind are generally known. They are used to convert an input voltage by means of a transformer into an output voltage electrically isolated therefrom. The input voltage can be e.g. a rectified line voltage or AC input voltage of a 50 Hz/230V or a 60 Hz/120V power supply network of a public utility company. The DC output voltage can lie in a voltage range of 1 V to 48 V. For automation applications it lies in particular in a voltage range of 15 V to 24 V.
The power supply devices under consideration have a load-dependent DC output voltage. In other words the closed-loop controlled DC output voltage decreases as the electrical load increases, i.e. as the output current increases. What is achieved by this means is that in particular similar power supply devices and power supply devices collectively feeding into a bus cable or busbar in accordance with a load distribution scheme are approximately equally loaded. Parallel infeeding is necessary in particular when the connected loads, such as e.g. automation components or system parts, are to continue operating without interruption in the event of failure of one of the power supply devices.
The voltage reduction can be taken into account in a closed-loop control system by suitable feedback of an output current value measured by means of a current measuring unit. Typically the DC output voltage, starting from a no-load voltage value, returns in a linear manner to a rated load voltage value at a rated output current. In the event of further loading of the power supply device in the sense of an overload or a short-circuit, the output current can be limited to the rated current.
The power supply devices under consideration are preferably designed as two-stage entities. The first stage is typically a step-up converter unit, which is preferably operated in a PFC (Power Factor Correction) operating mode. The second stage is a DC/DC converter, which converts the input voltage in an electrically isolated manner into the desired DC output voltage. Basically, different circuit topologies can be drawn upon for building the DC/DC converter. Thus, for example, the DC/DC converter can have a flyback converter, a forward converter, a push-pull converter, a resonant converter or the like. Depending on circuit topology and space requirements, the efficiency of a DC/DC converter of said type can be very different. As a general rule it holds that efficiency is at a maximum in a relatively narrow operating range of the DC/DC converter. In this case the pulse duty factor of a pulse-width-modulated switching element, in particular a switching transistor, plays a critical role. The pulse duty factor is in this case determined by the input voltage of the DC/DC converter, its output voltage and the transformation ratio of the transformer. For the considered power supply devices having what is termed a “soft” characteristic curve, i.e. having a load-dependent DC output voltage, this means that high efficiency values are only attained in a small part of the entire load range. The cause of this is the transformation losses of the transformer which are heavily dependent on the pulse duty factor.
In order to alleviate this problem it is known to over-dimension the DC/DC converter of the power supply device so as to achieve a higher efficiency overall.